Four common ways of depositing films are by chemical vapor deposition (CVD), plasma assisted/enhanced chemical vapor deposition (PACVD/PECVD), sputtering, and atomic layer deposition (ALD).
CVD is a process of deposition onto a substrate of a vapor phase reaction product of two vapor phase precursor compounds in a reaction chamber. The substrate is typically heated in order to enhance the deposition.
PACVD/PECVD is similar to CVD except that the two precursor compounds are ionized with application of a plasma. There are many benefits to using such a method.
A first benefit to employing a plasma in deposition is to crack relatively stable molecules and encourage deposition at much lower temperatures and pressures than would be required for thermal CVD.
A second benefit to using a plasma in deposition is more subtle but of great importance. Surfaces exposed to a plasma are subject to bombardment by energetic ions, whose kinetic energy can vary from a few eV to 100's of electron volts. Ion bombardment of this nature has very significant effects on the properties of the deposited film. Increasing ion bombardment tends to make films denser and cause the film stress to become more compressive. Denser films are especially desirable for dielectric films, because such films will have higher intrinsic dielectric value and better mechanical strength to withstand lattice mismatch strain and thermal expansion film stress. While excessive compressive stress can lead to impaired reliability, the compressive stress may be favorably adjusted in PACVD/PECVD through changes in process conditions, chamber geometry, or excitation (dual frequency mixtures).
A third benefit to using a plasma in deposition is the ability to deposit dense films at temperatures hundreds of degrees less than that required for thermal densification.
A final important benefit of using a plasma in deposition is the ability to easily clean the reactor. For example, by introducing a fluorine-containing gas (e.g. CF4) and igniting a plasma, one can clean silicon, silicon nitride, or silicon dioxide from the electrodes and chamber walls (albeit with rather more difficulty). Chamber cleaning is of great practical importance, because spalling of thick films built up on the parts of a chamber will create particles which can fall onto the substrates and cause defects in circuit patterns. As a result, the yield of good circuits from the process is reduced.
One example of depositing films containing oxides of germanium by PECVD is disclosed in Ultralow Loss High Delta Silica Germania Planar Waveguides, R. A. Bellman, G. Bourdon, G. Alibert, A. Beguin, E. Guiot, L. B. Simpson, P. Lehuede, L. Guiziou, E. LeGuen J. Electrochem. Soc., Volume 151, Issue 8, pp. G541-G547 (2004).
There are also many disadvantages to using PACVD/PECVD.
First, the bombardment of surfaces with ions can lead to undesirable sputtering, i.e., the displacement of atoms from the surface into the gas phase by incoming ions. These displaced ions diffuse through the gas and can land anywhere in the chamber, including on the substrates. So, sputtering can be an important source of trace metallic contamination in PECVD films.
Second, ion bombardment is different on horizontal and vertical surfaces, so the composition and density of films can depend on the topography.
Third, plasma deposition of metallic or other highly conductive films is challenging because the deposited film tends to short out the powered electrode of a capacitive plasma reactor, or coat the dielectric window of an inductive reactor thereby shielding the chamber from the magnetic field.
Fourth, the plasma generation apparatus contributes considerably to the complexity and cost of reactors. The plasma requires one or more power supplies, each with an appropriate matching network, and electrically insulating but mechanically sound materials for isolating powered electrodes. Substrate heaters must also be electrically isolated from the plasma.
Finally, use of a plasma in deposition is not desirable in some critical applications such as gate oxide deposition due to relatively greater sensitivity to contamination and process control considerations.
An alternative to CVD and ALD is sputtering. Sputtering is a vacuum process used to deposit very thin films on substrates for a wide variety of commercial and scientific purposes. It is often performed by applying a high voltage across a low-pressure gas (usually argon at about 5 millitorr) to create a plasma. During sputtering, energized plasma ions strike a target composed of the desired coating material and cause atoms from that target to be ejected with enough energy to travel to, and bond with, the substrate.
However, sputtering has many disadvantages. Glow discharge process is not an efficient form of ion production because 95 percent of the input power is dissipated as heat. In comparison to CVD, sputtering achieves relatively low deposition rates and low deposition rates lead to incorporation of impurities into the film. Moreover, fast moving secondary electrons bombard the substrate and thereby increasing the substrate temperature. This prohibits the use of this technique for highly temperature-sensitive substrates.
One example of a film comprising germanium oxide formed by sputtering is disclosed by U.S. published patent application no. 20040157354. In that method, aluminum is etched from the aluminum/germanium film and then oxidized to form a film containing germanium oxide.
Also, Thin Solid Films 189, 293, 1990, Bull Am Ceram Soc 45, 784, (1966) discloses that films of germanium oxides are typically prepared via a two step process involving deposition of germanium metal and then oxidization of the surface using either O2-containing plasma or ozone, or more directly by PACVD/PECVD.
In contrast to CVD, ALD is process in which two reactants, such as a metal or Group IV element precursor and a co-reactant, are alternatingly injected in gaseous form into a reaction chamber. Typically, ALD involves only one precursor and one co-reactant. The precursor is injected into the chamber and is chemisorbed upon the substrate surface, but cannot fully react with the substrate surface in the absence of the co-reactant. Next, a purge gas is injected into the chamber to remove all the unreacted precursor, and the reaction products from the chemisorption. Subsequently, the co-reactant is injected and a gas-surface reaction takes place on the substrate between the co-reactant and the substrate and/or the first precursor chemisorbed on the substrate. The reaction product is the desired film.
Each of the sequential precursor and co-reactant is injected in dosages sufficient to saturate the substrate surface. Because the precursor adsorption and the reaction between the co-reactant and the substrate and/or precursor is limited by the saturation limit of the substrate, further growth cannot occur without additional injections of precursor and co-reactant. Hence, thickness of the desired film is controlled by the number of times the precursors are introduced into the chamber, (otherwise known as precursor cycles), rather than the deposition time as is the case for conventional CVD processes.
It should be noted that the temperature of the ALD process is maintained in a range such that neither reactant is allowed to condense into multiple molecular layers or decompose on the surface.
An example of forming a film including hafnium oxide by ALD is disclosed by U.S. published patent application no. 20040175882.
Regardless of the deposition mechanism, hafnium oxide and composite films such as hafnium aluminates, silicates, and germinates have interesting semiconductor applications, such as in the fabrication of complementary metal-oxide semiconductor devices and capacitive memory devices. Whereas codeposition of aluminate and silicate has been demonstrated, the codeposition of germinates has not been demonstrated by non-plasma chemical vapor deposition which is desirable for some semiconductor applications.
Prior to this invention, methods of vapor depositing films containing germanium oxides without using a plasma or highly reactive oxidizers, like ozone, in the deposition process were not known.
Germanium oxides in particular could be very interesting high k film materials for future semiconductor fabrication. The deposition of germanium oxides is important for other applications such as infrared optical systems, optoelectronic devices, phosphors, and optical fibers.
Further, the incorporation of germanium oxide into silicon or other metal oxides offer wide range of novel compounds, which can offer unique properties useful in these applications. Thus, one of ordinary skill in the art will recognize the need for a new method of depositing films containing germanium oxides without requiring use of a plasma or oxidation by a strong oxidant such as ozone.